The present invention relates to a differential pressure control valve and a variable displacement compressor having same.
Japanese Laid-Open Patent Publication No. 2000-55223 discloses four examples of conventional differential pressure control valves. These differential pressure control valves are arranged in a flow passage that allows fluids from the upstream side to the downstream side with respect to the differential pressure control valve. Each differential pressure control valve has a valve seat, a valve body, and a guiding member. A valve hole through which fluid passes is formed in the valve seat. The valve body is located on the downstream side of the valve seat, and separates from or is seated on the valve seat in accordance with the pressure difference between the upstream side and the downstream side to open or close the valve hole. The guiding member is fixed to the valve seat to guide the valve body. In these differential pressure control valves, one of the valve seat and the valve body is made of magnetic material, and the other is made of a permanent magnet.
In these differential pressure control valves, when the valve hole is closed, the valve body remains seated on the valve seat due to the magnetic sticking force acting between the permanent magnet and the magnetic material. When the valve hole is opened, the valve body is urged toward the valve seat due to a magnetic pulling force (magnetic attraction) acting between the permanent magnet and the magnetic material.
In a general differential pressure control valve, which urges the valve body toward the valve seat using only a spring, the urging force of the spring increases as the distance of the valve body from the valve seat increases. In contrast, in the differential pressure control valve disclosed in Japanese Laid-Open Patent Publication No. 2000-55223, the magnetic pulling force of the permanent magnet decreases as the distance between the valve body and the valve seat increases. Thus, compared to a general differential pressure control valve, the valve seat is easily separated from the valve seat when the valve hole is opened. Therefore, even when the pressures difference is small, the differential pressure control valve of the publication reliably opens the valve hole to reduce the pressure loss when fluid flows from the upstream side to the downstream side via the valve hole.
However, in each differential pressure control valve of Japanese Laid-Open Patent Publication No. 2000-55223, the valve seat or the valve body is entirely made of a permanent magnet. Therefore, when the valve hole is closed, the permanent magnet collides with the magnetic material and is likely to be cracked.
In this respect, Japanese Laid-Open Patent Publication No. 2000-55223 discloses three modifications of differential pressure control valves. In these differential pressure control valves, a valve seat or a valve body that has a permanent magnet coated with resin is used. Since the resin alleviates the impact of collision between the permanent magnet and the magnetic material when the valve hole is closed, the permanent magnet is prevented from being cracked. This improves the durability of the differential pressure control valves.
However, in the differential pressure control valves according to the modified embodiments of Japanese Laid-Open Patent Publication No. 2000-55223, the resin exits between the permanent magnet and the magnetic material not only when the valve seat is seated on the valve seat, but also when the valve body is urged toward the valve seat. Since this hinders the flow of magnetic flux from the permanent magnet to the magnetic material, which reduces the magnetic flux density between the valve body and the valve seat. Accordingly, in these differential pressure control valves, the magnetic force acting between the valve body and the valve seat is relatively weak. This hinders the movement of the valve body toward the valve seat and prevents the valve body from being stably seated on the valve seat.
To solve the above problem, the permanent magnet may be, for example, increased in size without changing the type thereof. However, in this case, the permanent magnet is bulky and requires a relatively large space in the differential pressure control valve. This makes it difficult for the differential pressure control valve to be reduced in size. As a result, the variable displacement compressor having the differential pressure control valve is difficult to reduce in size.